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Lateral Geniculate Body and Visual Cortex

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Lateral Geniculate Body and Visual Cortex Vision, Accommodation & pupillary light reflex L4 DR.ABDUL MAJEED AL-DREES Objectives • Describe visual acuity • To know visual pathway and field of vision • Describe the process of accommodation reflex and its pathway, contrasting the refraction of light by the lens in near vision and in far vision • Identify and describe pupillary light reflex and its pathway and relate these to clinical situations as argyl Robertson pupil • Identify the lateral geniculate body and visual cortex VISUAL ACUITY • -degree to which the details of objects are percieved • -Snellens chart to measure visual acuity • Normal acuity = ( d/D = distance of Patient / distance of normal person = 6/6) • Visual threshold :is minimal amount of light that elicit sensation of light Visual Pathway 1. Optic nerve 2. Optic chiasm 3. Optic tract 4. Lateral geniculate body (nucleus) 5. Optic radiation 6. Visual cortex Visual Pathway • The optic nerves from the left and right eyes partially decussate in the optic chiasma – Fibers from nasal retina cross over – Leads to binocular vision • Left visual field viewed through right hemisphere • Travel through optic tracts to the Lateral Geniculate Nucleus (LGN) of the Thalamus LGB • Each layer receives input from one eye only: - Layers 1,4,6 from contralateral eye - Layers 2, 3,5 from ipsilateral eye • LGB also receives input from brain stem, reticular formation & feedback from cerebral cortex Lateral Geniculate Body; LGB • The Subcortical thalamic relay nucleus of vision, starting the process of co-ordinating vision from the two eyes • C-shaped 6 defined layers • Layers 1, 2 receive from large M ganglion cells → Magnocellular division • Layers 3,4,5 & 6 from small P ganglion cells → Parvocellular division Cortical Visual areas • - Primary (area 17) - Secondary association area, (areas 18, 19) Primary Visual area (Area 17) Brodmann's On medial aspect of each occipital lobe Its neurons arranged in the form of columns forming 6 distinct layers Fovea has broad presentation Visual Projection to Area 17 Role of Area 17 Perception of visible objects without knowing the meaning of these objects • Details of image; shape, borders & colours • Orientation of object in space • 3D vision (stereoscopic) Secondary Visual Processing: Association Areas (18 &19) • In parietal & temporal lobes • Interpretation of visual stimuli • Dealing with complex perception of patterns & forms responsible for object recognition • cells in large complex receptive fields & selective for specific stimuli (familiar faces) Retinotopic Organization & Processing of visual information Right visual field Left visual field Right Eye Left Eye Meyer’s loop Optic nerve Optic radiations Optic chiasm Optic tract parietal Lateral geniculate Right Visual cortex Calcarine fissure (area 17) Summary of visual projections Accomodation Focusing on a nearby object Accomodation Definition: Modification of the refractive power of the eye (curvature of the lens) to view a nearby object Goal: Clear vision of a nearby object Accomodation- cont. – Lens changes (accomodation) – Changes in the pupil – Convergence of the eyes The near response The Near Respose Image Focusing Fully relaxed (unaccommodated) • Lens accommodation Parallel light rays from Focal distant Distance light source Focal Distance Fully accommodated Accommodation When the cilary muscles are relaxed, the zonalus pulls tight and keeps the lens flattened for distant vision The elastic lens is attached to the circular cilary muscles by the zonalus which is made of inelastic fibres When the cilary muscles contract, it releases the tension on the zonalus and the elastic lens returns to a more rounded shape suitable for near vision Mechanism of accomodation Ciliary muscle ….Contraction: Relaxation of the suspensory ligament Lens more convex Increase diopteric power of the eye Near object focussed on the retina Mechanism of accomodation- cont Ciliary muscle ….Relaxation: Contraction of the suspensory ligament Lens less convex (Flat) Decrease diopteric power of the eye Far object focussed on the retina Distant Vision: Accommodation: Ciliary Muscle Relaxed Ciliary Muscle Contracts Suspensory Ligaments Under Reduced Tension on Suspensory Tension Lens is Flattened Ligaments Focus on Distant Objects Lens becomes Round Focus on Near Objects Accomodation-cont Lens changes during accomodation: – Affect the anterior surface of the lens mainly – Lens thickness increases Accomodation-cont Lens changes during accomodation: – Affect the anterior surface of the lens mainly – Lens thickness increases Diopter (D) Dioptre (s) = 1 Focal length (m) Diopteric power if the eye: Cornea …………40-45 D Lens …………… 15-20 D Accomodation …. +12 D Amplitude of Accomodation Definition The additional diopters added by increasing the convexity of the lens Near point: The nearest point to the eye where an object can be seen clearly Presbyobia: Loss of lens elasticity in old age >>loss of accomodation Near point and amplitude of accomodation Age (yrs) Near point (cm) Amplitude of Accomodation 10 9.0 11.0 20 10.0 10.0 30 12.5 8.0 40 18 5.5 60 83 1.2 70 100 1.0 The accomodation Reflex Afferent: Retina optic nerve optic chiasma optic tract lateral geniculate body visual cortex Efferent: Occuluomotor nucleus (parasympathetic) ciliary ganglion ciliary muscle circular pupillary muscle Convergence of the eyes • perception of two images as one • When the 2 images are not in register as one cortical neurons → excitation of interference cortical neurons → signals to Oculomotor apparatus → convergence or divergence or rotation to re-establish fusion Focusing on a nearby object The light reflex Light Reflex When an eye (Left) is subject to bright light, a direct light reflex occurs(constriction of the pupil) as well as a consensual (indirect) reflex of the other (Right) pupil The light reflex ==The constriction of the pupil in response to light Pathway: Retina optic tract superior colliculus occulomotor nucleus pupillary muscles Reflex arc: • light receptors optic nerve optic tract - pretectal area Edinger-Westphal nucleus - parasympathetic fibers of n. oculomotor n. ciliaris - m. sphincter pupillae decrease of pupillary diameter. • • Consensual pupillary light reflex: reaction of eye pupil to light irritation of opposite eye. It is possible due to diverging of nerve fibers from one pretectal nucleus to both Edinger- Westphal nuclei. The lateral geniculate body Note the pathway for pupillary contraction Constriction of the pupil The pupil constricts in response to: • The accomodation Reflex • The light reflex Argyll Robertson pupils (Neurosyphilis) Pupils constrict in response: to accomodation reflex but not to the light reflex Destruction of pretectal nucleus Argyll Robertson pupils • ARP, standing for Accomodation Reflex Present. Read it from back to front: it is PRA, standing for Pupillary Reflex Absent. The lateral geniculate body .
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